1. Field of the Invention
The present invention relates to an optical substrate for light emitting elements, a light emitting element using it, a display device using the light emitting element, and manufacturing methods thereof.
2. Description of the Related Art
Along with the advance of information and communication technology over the recent years, diverse display devices have been developed. Among them is the self-luminescent organic electroluminescence (EL) element, which is attracting interest for its high display quality and thinness.
The organic EL element is a self-luminescent element embodying the principle that recombination energy between holes injected from the anode and electrons injected from the cathode causes fluorescent substance to emit light by applying an electric fields. Since a report was published on a laminated low voltage-driven organic EL element, many research attempts have been made on organic EL elements composed of one organic material or another. One example is an element using tris(8-quinolinol) aluminum for the light emitting layer and a triphenyldiamine derivative for the hole transport layer. Advantages of the laminated structure include enhanced efficiencies of hole injection into the light emitting layer and of the generation of exciters, the latter being generated from a recombination by blocking the electrons injected from the cathode, and the enclosure of exciters generated within the light emitting layer. Other known element structures for organic EL elements such as the two-layered one cited above include a three-layered structure comprising a hole transport layer, a light emitting layer and an electron transport layer. These laminated elements embody many contrivances in element structure or formation method intended to enhance the efficiency of recombination of injected holes and electrons. Also, the wavelength of emitted light can be changed by using a different material for the light emitting layer.
However, in an organic EL element there is a constraint to the probability of singlet generation due to the dependence of spin statistics at the time of carrier recombination, resulting in an upper limit to light emitting efficiency. The level of this upper limit is known to be about 25%. When an iridium complex is used as the dopant material for the light emitting layer, luminescence from the triplet exciter of iridium arises at a high probability and this, combined with the utilization of the singlet exciter, enables exciter generation at a high probability of 75 to 100%.
Incidentally, organic EL elements and the like confine light by a total reflection effect as an optical phenomenon characteristic of them. As the refractive index of the light emitting layer or the transparent electrode is higher than that of the substrate or air, light whose angle of emission is at or above a critical angle is totally reflected by the transparent electrode/substrate interface or the substrate/air interface, and cannot be extracted out of the substrate. Supposing that the refractive index of the organic layer including the emitting layer is 1.6, that of the transparent electrode 2.0 and that of the substrate 1.5, the quantity of light emitted outside, namely the efficiency of light extraction is no more than 20% or so. For this reason, the limit of energy conversion efficiency is never high, only about 5% including the probability of singlet generation or, even if the triple exciter is utilized, no more than 15 to 20% in total. This poses a problem not only to organic EL elements but also to plane-light emitting elements in general, whose light emitting material discharges light.
As a method to enhance this light extraction efficiency, it is proposed in Patent Document 1 (Japanese Patent Laid-Open No. 2001-202827) to arrange a low refractive index layer between the substrate and the transparent electrode, and this laminated structure is shown in FIG. 11. According to this disclosed method, the presence of a transparent electroconductive film (transparent electrode layer) 302 in contact with at least one surface of a low refractive index body 301 serves to enhance the rate of extracting the light passing the low refractive index body 301 into the atmosphere and this enhanced rate of extracting the light outside, and the refractive index 1.003 to 1.300 of the low refractive index body 301 enable the light passing the low refractive index body 301 to be more efficiently extracted into the atmosphere, resulting in a higher extracting ratio of light to be extracted to the outside. Furthermore, an ultra-low refractive index close to 1 is realized by using silica aerogel for the low refractive index body 301.
Also, Patent Document 2 (Japanese Patent Laid-Open No. 2002-278477) discloses an invention by the same inventor in which the light emitting element of Patent Document 1 is applied to a thin film transistor (TFT) substrate. According to this disclosed technique, a low refractive index layer on the other side surface of the transparent electroconductive layer than the light emitting layer is within a range of 1.01 to 1.3.
Further, Non-Patent Document 1 (T. Tsutsui, Adv. Mater. 2001, 13, No. 15, August 3, pp. 1149-1152) discloses a structure in which an organic EL element is provided over a substrate having a silica aerogel film of 10 μm in thickness arranged as the low refractive index layer, a silicon oxide (SiO2) film of 50 nm in thickness arranged over the silica aerogel film, and a transparent electrode (ITO) of 100 nm in thickness arranged over the silicon oxide film. The silicon oxide film here is formed by sputtering. The reference states that the external quantum efficient of the disclosed structure is 1.8 times higher than that of an element structure having no silica aerogel film.
However, the above related arts leave room for improvement in the following respects.
The structures described in Patent Documents 1 and 2, in which a low refractive index layer is arranged between the substrate and the transparent electrode layer, are effective in that the efficiency of light extraction is enhanced by collecting the light within a critical angle, but the reflection of light by the interface between the transparent electrode and the low refractive index layer makes the enhancement of light extraction efficiency still insufficient. When a porous silica aerogel film is used to obtain an ultra-low refractive index layer, the mechanical strength of the film is extremely weak. Further, when the transparent electrode is patterned in a wet process, the etchant flowing round from the porous silica aerogel film makes it difficult to form the prescribed pattern. Moreover, the surface roughness of the porous film invites inter-electrode leaks and pixel short, giving rise to unstable light emission or failure to emit light. Thus, these techniques are still inadequate as light extraction methods that can be applied to organic EL elements.
The method described in Non-Patent Document 1 by which a silicon oxide film is formed by sputtering over a silica aerogel film of 10 μm in thickness is also inadequate in that the sputtered film does not serve to improve surface roughness of the silica aerogel film, and similarly invites inter-electrode leaks and pixel short. Further it is difficult to form a silica aerogel film as thick as 10 μm with sufficient uniformity, and accordingly element characteristics are susceptible to fluctuation. If this method is to be applied to a display device provided with a thin film transistor (TFT), the 10 μm film thickness will make it difficult to form contact holes needed for connecting the pixel electrode and the source electrode of the TFT, making the intended application impossible.